A 69-year-old man with long-standing hypertension and congestive heart failure presents with rising creatinine from 120 to 220 µmol/L over 10 days and potassium 6.1 mmol/L. Medications include ramipril, spironolactone, and furosemide. He is clinically euvolaemic and blood pressure is 110/70 mm Hg. What is the most appropriate initial step in managing his acute kidney injury and hyperkalaemia?